Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WO 94/28242 2 1 6 3 3 7 0 PCT/SE94/00463
A METHOD OF FORMING A TISSUE PAPER WEB
TECHNICAL ~ LD
The present invention relates to a method of forming a tissue paper web in a twin wire
former having a rotatable forming roll, said method including the steps of: injecting a jet
consisting ç~senti~lly of an aqueous slurry of p~renn~king fibers into a converging forming
throat formed between two looped forming fabrics as they first co~lver~e to meet on a periphery
of the rotatable forming roll and then partially wrap the forming roll periphery; sandwiching the
aqueous slurry between the two forming fabrics and tl~ining water from the slurry through at
0 least one of them as they partially wrap the forming roll ~flph~,.y; co.,l;.ll.i~g the draining, and
(1r~ining a sufficient proportion of water from the slurry to cause the p~r~nn~king fibers to
form a fibrous web; running the two forming fabrics with the papçrm~king fibers sandwiched
between them up to and around a section of a second roll; and sepal~tillg one of the two
forming fabrics from the formed fibrous web and the other forming fabric no earlier than on
said second roll.
In this context, the term "tissue paper" is int~n(lPd to include any grade of "soft crepe
paper" or other paper for sanitary ~u~Joses, whether creped or not when used by a consumer.
BACKGROUND OF THE INVENTION
Such a method is disclosed in US-A- 4,100,018 (Wahren et al.), for example, and is the
method inherently used for forming a tissue paper web in a PERIFORMER~9-LW forming
section of S-wrap configuration. Due to the rapid drainage of water from the slurry in high
speed twin wire formers for tissue making (about 1800 to 2000 m/min or higher with a
PERIFORMER'I9-LW forming section of C-wrap configuration), there is almost no time at all
25 for the pzlpennslking fibers to rearrange themselves or for the paper producer to exert an
infll-enre on the formation of the web, that is the manner in which the fibers are distributed,
arranged and mixed in the structure of the web. Thus, the formation of a web produced in a
twin wire former could be ch~ teri7çd as "frozen" and reflects the relative positions of the
fibers in the stock just before the drainage starts. Any disLulbancc or hllpt;lr~lion at this point
30 will inevitably be found in the web formed on the draining of the slurry. An excellent
distribution of the fibers in the slurry will result in an ~rellent formation, while a less perfect
fiber distribution also will result in a less perfect formation and may appear as pin holes or
streaking, for example.
In twin wire formers for the production of n~,~. Sylillt and other printing paper grades,
3s such as SPEED-FORMER HS by Valmet Paper Machinery Inc., and the one for the production
WO 94/28242 2 1 6 3 3 7 ~ 2 PCT/SE94/00463
of lightweight coated grades of paper (LWC) disclosed in US-A- 4,790,909 (Harwood), the
- speeds used are much lower, about 1300 to 1500 m/min and about 900 to 1050 m/min,
,espe~;~iv~ly. In these cases the web produced should have as close to one-sided surface
properties as possible, that is a minimllm of two-si(lP-ln~s, and the retention of fines and fillers
s in the surfaces of the web should be co~l~p~ble to that obtained in a fourdrinier former.
PULP AND PAPER, December 1982,J. C. W. EVANS "New twin wire former dçsign~d
for m~ximl-m fines, solids retention", page 58, discloses a modified new design of the Bel-Baie
II twin wire former. The new former, called Bel-Baie m, is reported to be (ltDcign~l to retain
the formation of the Bel-Baie II and offer improved solids retention. It is also reported that the
0 Bel-Baie II design is still recomm~n-lçd for all p~ king operations other than those using a
high-fines furnish, or for tissue-ma_ing where the twin wire tissue former is lJIGrell~,d.
DISCLOSURE OF THE INVENTION
The main object of the present invention is to provide a method of forming a tissue web
5 having improved formation and improved tensile properties without any appreciable
deterioration of retention in comparison to tissue webs produced in conventional twin wire
tissue forrners.
According to the present invention this object is achieved, in a method of the k-ind
initially stated, by draining substantially all drainable water from the slurry while in a zone
20 curving along the pG..ph~ of the forrning roll up to where the two forming fabrics run off
from the forming roll periphery, but leaving a sufficient proportion of drainable water to have a
substantial arnount of the p~perm~king fibers free in the slurry during an initial phase of a
subsequent step, and draining, dowl~ c;a~ of said zone, the left proportion of drainable water
from the slurry while vibrating the slurry sufficiently to create a micro-turbulence causing a
2s small scale agitation of the fibers to prevent them from forming any appreciable fibrous web
until the water rçm~ining in the slurry is insufficient for allowing the fibers to ~.ubs~ltially
change their position relative to one another.
In this context, the term "drainable water" is inten-led to mean the water that can be
drained from the sandwiched slurry in the twin wire former by the use of col~c;--Lional web
30 forming technology. Even when all drainable water has been drained off, the newly formed
paper web on leaving the web forrner may still have a moisture content of 85 %, for example.
By tlr~ining all of the drainable water at the forming roll, with the exception of a minor
fraction left to permit a s~lhst~nti~l amount of the fibers to be free to move on a small scale in
the slurry and rearrange themselves under exposure to the vibrations, web formation is
3s improved and, surprisingly, retention is comparable to the one obtained in a conventional twin
WO 94/28242 2 1 6 3 3 7 0 PCT/SE94/00463
wire roll former instead of deteriorating due to the vibrations, as is the case in twin wire blade
formers for the production of n~w~h~t and other printing paper grades.
Preferably, the micro-turbulence is achieved by vibrating the slurry at a frequency of at
least 100 Hz. Then, the paperrn~king fibers in the slurry have no time to build an embryonic
5 web on each of the two forming fabrics, and--at least in theory--it should be possible to
achieve a total sheet set with the best possible formation by completing the draining of the
slurry while the fibers are kept consL~,ILly agitated until there is not sufficient water left to
permit agitation.
In order to obtain vibrations of a small amplitude for creating the micro-turbulence, we
o prefer to provide a multiblade hydrofoil at a location dowll~LIealll of the forming roll but
u~LI~;aul of the second roll, and to direct one of the forming fabrics across the foil blades of
the multiblade hydl~r~il. The hydrofoil has a plurality of eqnicli~t~nt1y spaced foil blades of
equal size, suitably at least four foil blades disposed on a center to center spacing on the order
of 50 to 330 mm, for contacting a contiguous one of the forming fabrics, and defines a
5 substantially convexly curved surface ~u~olli11g said one forrning fabric.
Although the in~t~ tion of a multiblade hydrofoil in accol~lce with the present
invention may be applied in various configurations of twin wire tissue formers, we prefer
starting from a twin wire former of basically C-wrap type and disposing the multiblade
hydrofoil within the loop of the forming fabric that con~liluLes an outer forming fabric relative
20 to the forming roll and the other forming fabric, while the second roll being disposed within the
loop of the inner forming fabric.
It is also preferred to provide as forming roll a suction forming roll, and to drain the
slurry through both of the forming fabrics in the zone where they wrap the forming roll. In
comparison with a plain-snrf~ed forming roll, a suction forming roll will contribute to
2s i~ ovt;d formation at medium and high basis weights. This effect is more pronounced with
recycled fibers and also with increasing basis weight. The wrap angle of the outer forming
fabric on the suction forming roll suitably is on the order of 15~ to 45~, while on a
plain-sllrf~e~ forming roll it would be on the order of 45~ to 135~.
In order to have an u~Lilllulll amount (suitably from about 1 % to about 10 %, preferably
30 about 1 % to about 2 %) of drainable water left in the slurry when the partially drained slurry
sandwiched between the two forming fabrics arrives at the multiblade hydrofoil, we prefer to
provide a headbox for discharging the slurry into the forming throat, provide a breast roll for
the outer forrning fabric imme~ t~ly upstream of the forming throat, and swing the headbox
and the breast roll as one assembly around a rotational axis of the forming roll to adjust the
WO 94/28242 2 ~ PCT/SE94/00463
degree of wrap of the outer forming fabric on the forming roll, and thereby also adjust the
proportion of drainable water drained from the slurry at the forming roll.
BRIEF DESCRIPTION OF THE DRAWINGS
s Fig. 1 is a s~hPm~tic side elevational view of a roll type twin wire tissue former modified
into a roll-and-blade former by the inct~ll,.tion of a multiblade hydrofoil in accc,..la"ce with one
embodiment of the present invention.
Fig. 2 is an enlarged scale side elevational view of the multiblade hydrofoil shown in
Fig.l.
IO Fig. 3 is a graph illnctr~ting the i~ uv~ ent in formation of virgin fiber webs produced
at various speed differentials in the roll-and-blade former of Fig. 1 over those produced under
similar conditions in two prior art C-wrap formers having an open-sllrf~ed forming roll and a
plain-surfaced one, ~c~yeclively.
Fig. 4 is a graph illllctr~ting the improvement in formation as a function of MD/CD
tensile ratio in virgin fiber u~ Gped webs produced in the roll-and-blade former of Fig. 1 over
those produced under similar conditions in two prior art C-wrap formers having an
open-sllrf~red forming roll and a plain-snrf~red one, Ic~ ;lively.
Figs. 5 and 6 are graphs similar to Figs. 3 and 4, I~ e~;livcly, but with recycled fibers
~ub~.liluled for the virgin fibers.
Fig. 7 is a graph illustrating the hll~luv~llent in formation as a function of basis weight
of virgin fiber uncreped webs produced in the roll-and-blade former of Fig. 1 over those
produced under similar conditions in two prior art C-wrap formers having an open-sllrf~ed
forming roll and a plain-sllrf~red one, Ics~ecliv~ly.
Fig. 8 is a graph illustrating the hll~luvclllent in tensile efficiency of virgin fiber creped
webs produced at various speed lirrclclllials in the roll-and-blade former of Fig. 1 over those
produced under similar conditions in a prior art C-wrap former having an open-snrf~ced
forming roll.
Fig. 9 is a graph illustrating the improvement in tensile efficiency as a function of
MD/CD tensile ratio in virgin fiber uncreped webs produced in the roll-and-blade former of
Fig. 1 over those produced under similar conditions in a prior art C-wrap former having an
open-sllrfared forming roll.
Fig. 10 is a graph illllctr~ting the layer purity in a three-layer web produced in the
roll-and-blade former of Fig. 1.
WO 94/28242 2 1 6 3 3 7 0 PCT/SE94/00463
S
Figs. 11, 12, 13 and 14 are schematic side elevational views of four additional roll type
twin wire tissue formers modified into roll-and-blade formers by the in~t~ tinn of a multiblade
hy~ roil in accordance with the present invention.
s DETAILED DESCRIPrION OF THE MOST PREFERRED EMBODIMENT
Basically, the former illustrated in Fig. 1 is a cullvelllional roll type twin wire tissue
former that has been modified in some respects in view of the method steps carried out, in
accordance with the present invention, for forming a tissue paper web W in a twin wire former
having a rotatable forming roll 1. A known method of forming a tissue paper web in a
0 conventional roll type twin wire tissue former includes the following steps:a) Injecting from a headbox 2 a jet cull~i~tillg ess~onti~lly of an aqueous slurry of
p~rçrrn~king fibers in water into a cullvcl~,illg forrning throat S formed between two looped
forrning fabrics 3 and 4 as they first converge to meet on a pcfi~hc~y of the rotatable forming
roll 1 and then partially wrap the forming roll periphery. Fabric 3 con~tit~-tçs an outer forming
fabric in relation to fabric 4 in a zone where the fabrics partially wrap a portion of the forming
roll periphery, and forming roll 1 is located inside the loop of the inner forrning fabric 4. The
illn$trat~ci headbox 2 is a multilayer headbox for dischdl~ g a multilayered jet of stock into the
forming throat 5, more precisely a three-layer headbox, but it might as well be a two-layer
headbox or a single-layer headbox.
b) Sandwiching the aqueous slurry between the two forming fabrics 3 and 4, and
draining water from the slurry through at least one of them as they partially wrap the forming
roll periphery.
c) Continlling the draining, and draining a sufficient proportion of water from the
slurry to cause the ~ .. "~king fibers to form a fibrous web W.
2s d) Running the two forming fabrics 3 and 4 with the p~pl-rrn~king fibers sandwiched
between them up to and around a section of a second roll 6.
e) Separating one of the two forming fabrics 3 and 4 from the formed fibrous web W
and the other forming fabric no earlier than on said second roll 6.
In order to provide a method of forming a tissue web having hll~ vcd formation and
improved tensile properties without any ap~lcciable deterioration of retention and layer purity,
if applicable, in comr~ri~on to tissue webs produced in collventional twin wire tissue formers,
the known method above is supplemented with the following steps in accolJ~lce with the
present invention:
f) Draining s~ bst~nti~lly all drainable water from the slurry while in a zone Z curving
3s along the periphery of the forming roll 1 up to where the two forming fabrics 3 and 4 run off
WO 94/Z8242 2 1 6 3 3 7 ~ PCT/SE94/00463
from the forming roll periphery, but leaving a sufficient proportion of drainable water to have a
substantial amount of the p~perrn~king fibers free in the slurry during an initial phase of step
g); and
g) Draining, d~wllslrt;am of said zone Z, the left proportion of drainable water from
5 the slurry while vibrating the slurry sufficiently to create a micro-turbulence causing a small
scale agitation of the fibers to prevent them from forming any appreciable fibrous web until the
water rem~ining in the slurry is insufficient for allowing the fibers to ~.I,sli...l;rlly change their
position relative to one another.
Preferably, the micro-turbulence is achieved by vibrating the slurry at a frequency of at
o least 100 Hz. Then, the p~pçrrn~king fibers in the slurry have no time to build an embryonic
web on each of the two forming fabrics, and--at least in theory--it should be possible to
achieve a total sheet set with the best possible formation by completing the ~lrrining of the
slurry while the fibers are kept constantly agitated until there is not sufficient water left to
permit agitation.
s In order to obtain vibrations of a small r.mplit~lcle for creating the micro-turbulence, we
prefer to provide a multiblade hydrofoil 7 at a location dOwll~.LI~,a.l. of the forming roll 1 but
Llc;alll of the second roll 6 as shown in the embodiment illn~trrt~d in Fig. 1. There the
hydrofoil 7 is located inside the loop of the forming fabric that con~.LiLul~s the outer forming
fabric 3 in relation to the forming roll 1 and the other forming fabric 4 in the twin wire former,
while the second roll 6 is located inside the loop of the inner forming fabric 4. The 'hydl~roil 7,
which is shown on a greater scale in Fig. 2, has a plurality of elongate eqni~ t~ntly spaced
wear-resistant foil elements 8, which are arranged in a side by side relationship and extend over
the width of the forming fabrics 3 and 4. The vibrations of small amplitude are obtained by
directing one of the forming fabrics, in the embodiment illnctr~ted in Fig. 1 the outer forming
2s fabric 3, across the foil element~ 8. Suitably there are at least four foil elçm~nt~ 8 of equal size
disposed on a center to center spacing on the order of 50 to 330 mm, for contacting a
contiguous one 3 of the forming fabrics 3 and 4, and they are located so as to define a
substantially convexly curved surface s~ polLillg said one forming fabric 3. In view of t;v~lyday
language, the foil elçm~ntc 8 h~,llccrulL}I will be referred to as foil blades i~ ;Liv~ of their
actual shape, which may be similar to a substantially square rod, for example.
In the preferred embodiment illll~tr~ted most clearly in Fig. 2, the hydrofoil 7 has nine
foil blades 8 mounted on a box-shaped carrier 9 to form the convexly curved support surface,
which may have a radius of curvature on the order of 5 m. The shown foil blades 8 are of a
basically rectangular cross sectional shape and have a width on the order of 50 mm with the
3s exception of the leading foil blade, which is e~rt.-n(l~d on its leading side to form an edge
WO 94/28242 2 1 6 3 3 7 0 PCT/SE94/00463
defined by a top surface and a leading lateral surface of the foil blade and enclosing an edge
angle on the order of 45~. The leading as well as the trailing lateral surfaces of the other foil
blades have top portions sloping inward at angles on the order of 15~, so that the formed
enclosed edge angle will be on the order of 75~, and so that said other foil blades will be
5 symmPtn~ ~l All of the foil blades 8 have a top surface that forms a part of the curved fabric
~u~polLhlg surface. This top surface is provided with a longitll-lin~lly e~t~ntling crest located
about 25 mm from the trailing edge of the foil blade, and from this crest both the leading and
the trailing portions of the top surface slope dowllw~u.;ls at a small angle, preferably on the
order of 0.25~.
By means of a partition 10 the carrier 9 for the foil blades 8 is divided into a leading
col"pa,l"~ent 11 and a trailing co",pa L"",l~l 12, both of which are provided with outlets 13 and
14, l~;sl~e~;lively, for water drained from the sandwiched slurry at the end of the web forming
step. Both of the c~""pa.L",ents are also provided with stub pipes 15 and 16, ~c~e~;lively, for
connection over suitable p-~,S~ulc; controlling means, not shown, to a vacuum system, also not
shown, for ~c~i~ting in the rlr~inin~. Stub pipe 16 is mounted in a top wall of trailing
compartment 12 out of reach for possible splashes of drained water, while stub pipe 15 is
mounted in a rear wall of leading co""~d,l",ent 11 and is protected from splashing water by a
shield plate 17 interposed between the stub pipe and the foil blades. Alternatively, the vacuum
system is provided with a water sepa,dlu., not shown, and is coml~led to the outlets 13 and
14. At its top, the carrier 9 is pivotally hinged in two brackets 18, one of which is shown in
Fig. l, and the brackets are adjustably mounted to a frame member 19 of a framework for the
twin wire former. Spaced from the two brackets 18 there are provided two carrier position
adjusting means, one of which is shown as a rod 20 having one end pivotally co.ule~;led to the
carrier 9 and the other end adjustably colll,e~ ,d to another frame member 21 of the framework.
In the p,~efG ,~d embodiment ill~ d in Fig. 1, the forming roll is a suction forming
roll 1, and a breast roll 22 for the outer forming fabric 3 is located inside the forming fabric
loop in such a position in relation to the forming roll 1 and the multiblade hydrofoil 7 that the
outer forming fabric 3 will wrap the pe~iphG~y of the suction forming roll 1 over an angle a on
the order of 15~ to 45~. While a plain-s~ red forming roll may be used, it is less l"t;rt;".,d and
it would require a wrap angle a on the order of 45~ to 135~. I-~espe-;live of what type of
forming roll is being used, in cases where a broad range of basis weights is to be produced on
the roll-and-blade former according to the invention it may be advantageous to design the
former so as to permit a swinging of the headbox 2 and the breast roll 22 as one assembly
around a rotational axis of the forming roll 1. Thereby it will be possible to adjust the degree of
wrap (angle a) of the outer forming fabric 3 on the forming roll 1 and, consequently, also
WO 94/28242 2 1 6 3 3 7 ~ PCT/SEg4/00463
control the proportion of drainable water drained from the slurry at the forming roll 1. Suitably,
from about 90 % to about 99 %, preferably about 98 % to about 99 %, of the drainable water
should be drained from the slurry at the forming roll 1, so that not more than from about 1 %
to about 10 %, preferably about 1 % to about 2 %, of the drainable water remains in the slurry
5 on its arrival at the multiblade hydrofoil 7. This small rem~ining amount of drainable water will
result in the advantage that there will be no deterioration in retention andlor in layer purity, yet
it will facilitate the achievement of a sudden "total sheet set" (i. e. in principle no formation of
embryonic webs on the forming fabrics 3 and 4 with slurry of a lower consistency sandwiched
between them), which we believe requires a continuous small scale agitation of the fibers, until
lo the water rem~ining in the slurry is in~llffi~ nt for allowing the fibers to ~ubsL~Illially change
their position relative to one another.
After the forming of the web W at the multiblade hydlv~il 7 the web runs sandwiched
between the two forming fabrics up to and around the second roll 6. This roll is shown as being
a plain-sllrfa ed roll but, if desired, it might as well be a suction roll to assist in the removal of
5 water from the web W. In the embodiment illustrated in Fig. 1, a plain surface on the second
roll 6 gives the advantageous "table roll effect", which will assist in arlhering the web W to the
inner forrning fabric 4 when the outer forming fabric 3 on leaving the second roll 6 is defle~trd
a small angle from the inner forming fabric 4 and the web W carried thereby. To ensure the
desired transfer of the web W to the inner forming fabric 4, a transfer suction box 23 may be
20 provided downstream of the second roll 6 inside the inner forming fabric loop.
In order not to l"",r~es~.;ly crowd the drawings, some al,pal~Lus, which constitutes no
part of the present invention but is n~x~e.~";., y or advantageous for running a twin wire former, is
not disclosed. As an example, there is a first save-all, not shown, located between breast roll 22
and hydrofoil 7 for collecting white water drained through the outer forming fabric 3 in zone Z
2s and thrown outward from the forming roll 1. Further, on the opposite side of the inner forming
fabric 4 there is a white water deflector, not shown, mounted in a closely spaced relationship to
the inner forming fabric 4 immetli~ely dowll~L eam of the point where the forming fabrics run
away from the forming roll 1. The deflector is provided with a curved extension r~ le~ g over
the top of the forming roll 1 to a second save-all, not shown, located on the right-hand side of
30 forming roll 1 in Fig. 1 for collecting white water that has passed through the inner forming
fabric 4 into the suction forming roll 1, where it has been temporarily stored until the forming
fabrics leave the suction zone.
Figs. 3 to 10 ilhl~t~ the improvement in some relevant properties of a tissue paper web
formed in accc.l.lance with the present invention. In Figs. 3 to 9 open square dots (~) represent
35 measured values relating to tissue webs formed in a roll-and-blade twin wire tissue former of
WO 94/28242 2 1 6 3 3 7 0 PCT/SE94/00463
the kind shown in Fig. 1, while solid square dots (-) and solid delta dots (~ ,plesent measured
values relating to webs formed in a conv~nlional C-wrap twin wire tissue former having a
suction forming roll and a plain-surfaced forming roll, ~ Jeclively.
The difference between the speed (Vjet) of the stock jet ejected from the headbox 2 and
s the speed (VWj~e) of the forming fabrics 3 and 4 affects the formation of the web W. Figs. 3 and
4 are graphs illustrating the variations in "beta formation" with varying speed dirr~ ials and
varying MD/CD tensile ratios".,~.~eclively, and all beta formation values that we refer to are
measured on u"creped web samples.
The term "beta formation" means standard deviation in basis weight as measured by beta
0 radiation. ~n~equPntly, a low beta formation value is better than a high one. A suitable
instrument for me~uring beta formation is the AMBERTEC Beta Formation Tester available
from Ambertec Oy, Espoo, Finland.
The term "MD/CD tensile ratio" means the tensile strength of a web in its lengthdirection (i. e. the m~-~himP direction) divided by that in the cross direction of the web. The
5 tensile strength tests were carried out in accor~lce with standardized test ~luccdul~; TAPPI
T-494 (SCAN-P 44:81).
In Fig. 3 most of the dots are provided with a two-digit decimal number hldic~ltillg the
MD/CD tensile ratio of a test web of a specified beta formation and produced at a specified
speed dirr~.t;lllial in a roll-and-blade former, a C-wrap former having a suction forming roll,
20 and a C-wrap former having a plain-snrf~red forming roll. The test values in Fig. 4 relate to
uncl.,ped webs, while the MD/CD values in Fig. 3 relate to creped webs. In other respects the
webs having the test results i~ tr~t~d in Figs. 3 and 4 are identir~l and consist e~Pn~i~lly of
virgin fibers and have a basis weight of 20 g/m2. All basis weights that we refer to are
measured on u~l~u~ped web s~mple~, and the virgin fibers were 50 % Sç~.,.l;" ~vian softwood
2s and 50 % eucalyptus, and the webs were prepared from a stock having a freeness value of
about 600 CSF. Freeness value, or CSF-number (c~n~ n Standard Freeness), is a measure of
the drainability of the stock and is det-~rrninP~ accor lhlg to standardized test pluce lu~s, e. g.
TAPPI T-227 (SCAN-C 21 or SCAN-M 4).
The webs, on which the test results illll~tr~t~d in Figs. 5 and 6 are based, differ from
30 those in Figs. 3 and 4 only in consisting e~Pn~i~lly of recycled fibers from computer print-out.
The stock of recycled fibers had a freeness value of about 250 CSF.
Figs. 3 to 6 clearly show that in comparison with tissue webs formed in conventional
twin wire roll type formers, where as a rule a not very ~ti~f~tory formation has to be accepted
in case high MD/CD tensile ratios are desired, webs formed in acco.dance with the present
3s invention m~int~in a very ~ti~f~rtnry formation even at high tensile ratios. As illustrated in
WO 94/28242 2 1 6 3 3 7 0 PCT/SE94/00163
Figs. 3 and 5, the beta formation of a web formed on the roll-and-blade former, as contrasted to
that of a web formed in a C-wrap type former having a suction forming roll or a plain-sllrf~ed
forming roll, is s~lbst~nti~lly constant at speed differentials (Vjet - VWire) on the order of from
about-200 m/min to about +250 m/min. The possibility of forming tissue webs that in addition
5 to very ~:~ti~f~t--ry formation have high MD/CD tensile ratios, i. e. tensile ratios in the range of
2 - 5, is of great interest in m~nnf~ tllring a majority of various tissue products, and the
utili7~tinn of speed differentials is the prevailing method of obtaining high MD/CD tensile
ratios. A comparison of Figs. 3 and 4 with Figs. S and 6 clearly in~ tf s that in case the webs
are formed from recycled fibers instead of virgin fibers the advantages of the present invention
0 over prior art methods are still more pronounced. Consequently, the method of the present
invention may also be c!~ lf ;~r~ as being very i~.c~ ive to changes in freeness value.
Fig. 7 is a graph illustrating the slight increase in beta formation with i.lcl~ash~g basis
weight of a web formed on the roll-and-blade former, as contrasted to that of a web formed in a
C-wrap type former having a suction forming roll or a plain-snrf~red forming roll. As pointed
5 out above, beta formation is the standard deviation in basis weight, and a low value is better
than a high one. The beta form~tion was measured on uncreped webs having an MD/CD tensile
ratio of 2 to 4 after creping and consisting f ~çnti~lly of virgin fibers. Even at a basis weight of
about 28 g/m2, a web formed in accolddllce with the present invention on a roll-and-blade
former has a beta formation that is better than that of a web of a basis weight of about 20 ~/m2
20 formed on a conventional C-wrap former having a suction forrning roll. As is obvious from Fig.
7, the method according to the present invention is advantageous over large basis weight range.
Figs. 8 and 9 are graphs illn~trating the variations in tensile efficiency with varying speed
dir~l~"lials and varying MD/CD tensile ratios, it;s~,~;Lively. The term "tensile efficiency"
means the "tensile index" of the web sample expressed as a percentage of that of a laboratory
2s sheet prepared from machine chest stock in acco,dallce with standardized test procedure TAPPI
T-205 (SCAN-C 26:76 or SCAN-M 5:76), and the process for clçt~rrnining the tensile index is
described in TAPPI T-220 (SCAN-C 28:76 or SCAN-M 8:76). The test results ill1l~tr~tPd in
Figs. 8 and 9 relate to samples of tissue webs consisting ~ssçnfi~lly of virgin fibers and having
a basis weight of 20 g/m2, and the tensile efficiency values are measured on uncreped web
30 samples. Also in this case the open square dots represent measured values relating to tissue
webs formed in a roll-and-blade twin wire tissue former of the kind shown in Fig. l, while the
solid square dots represent measured values relating to webs formed in a co,l~ ional C-wrap
twin wire tissue former having a suction forming roll, and the two-digit decimal numbers
against the individual dots indicate the MD/CD tensile ratios after creping of the various web
35 samples. As is evident from Figs. 8 and 9, the improvement in tensile efficiency of webs
WO 94/28242 2 1 6 3 3 7 0 PCTISE94/00463
11
formed in acco~dallce with the present invention over those forrned in a conventional C-wrap
twin wire tissue former having a suction forming roll is ~ul,sl .~
Fig. 10 is a graph illllct~ting layer purity of a three-layer web having a basis weight of
22.5 g/m2 and formed in accol.lallce with the present invention. The basis weight split is 30 %
s hardwood, 40 % softwood, and 30 % hardwood (eucalyptus). Contrary to what could be
expected, the layer purity is fully colllpal~ble to the one obtained in a collv~l,lional twin wire
roll type tissue former.
Sl-mm~ri7ing the advantages of the forming method according to the present invention
we get the following:
Form~tion
As demonstrated above, an improved formation can be achieved. A good formation is a
prerequisite for achieving the desired softness of the web, and for achieving a uniform
permeability of the web. A uniform permeability is çssçnti~l when using through air drying
(TAD) technology for drying the web. Further, an hll~luvc;d formation results in an hll~lovtd
5 runnability of the tissue machine, since also the ullirolllliLy of the yankee dryer coating will be
improved.
Alternatively, the ability of the roll-and-blade former to illl~l~ Vt~ the formation can be
utilized to m~int~in an already s~ti~f;~ctory formation and to start the forming of the web by
ejecting a stock jet of a consisL~ y higher than usual from a headbox having a slice opening of
20 reduced gap width. The use of a higher consistency means that less water will have to be
drained from the stock to form the web, and less energy for pumping will be required.
Formation/Tensile Ratio
As stated above, the beta formation of a web formed on the roll-and-blade former is
substantially constant at speed dirr~ ..,IlLials (Vjet - Vwjre) on the order of from about -200 m/min
2s to about +250 m/min. The utilization of speed differentials is the prevailing method of
o'~hling high MD/CD tensile ratios. The possibility of forming tissue webs that in addition to
very s~ticf~rto;y formation have high tensile ratios, i. e. tensile ratios in the range of 2 - 5, is of
great interest in m~nl-f~-~tming a majority of various tissue products and is a major advantage of
the present invention. On conventional twin wire roll formers the formation starts to deteriorate
30 already at lower tensile ratios, so that it is necessary to accept a less good formation in order to
reach the desired high tensile ratios.
Tensile Strength
An hllplov~d formation always gen.or~s a higher tensile strength. The reason therefor is
that the fibers are utilized more efficiently. Higher tensile strength means higher tensile
35 efficiency, which, if desired, can be used for reducing the refining of the pulp or the proportion
WO 94128242 2 1 6 3 3 7 0 12 PCT/SE94/00463
of long fibers in the stock, and a softer web of higher quality can be achieved. Less refining
- also means improved draining and drying capacities of the tissue machine.
Basis Weight Range
The method in accordance with the present invention of forrning a web on a
s roll-and-blade former enables a tissue m~mlf~rtllrer to produce high quality tissue paper within
a very large range of basis weights. One design of the multiblade hydrofoil, such as the
nine-blade two-compartment hydrofoil illllctr~ted in Fig. 2, is sufficient for permitting the
forming of webs having basis weights ranging from about 13 g/m2 to about 50 g/m2. Above
50 g/m2 it is recommendable to add a compa~ L with additional blades, and with un~ ,ed
0 webs having basis weights lower than about 13 g/m2 you have the problems of forrnation of
pinholes in the web like in con~.,.llional suction roll formers.
The forming method according to the present invention is very i~ , to changes infreeness value. The advantages referred to above are achieved when the paperm~king fibers
consist ~enti~lly of virgin fibers as when they consist çssçnti~lly of recycled fibers. In fact,
the advantages achieved when the webs are formed from recycled fibers appear to be more
pronounced than when they are formed from virgin fibers.
Multilayering and Retention
The roll-and-blade former used for carrying out the method in acco.dance with the
present invention ~ ly gr~ c as good layer purities and retention levels as does the
convt;"Lional genuine roll former. The reason herefor is that i~ e.,Liv~ of the in~t~ tinn of
the multiblade hydrofoil we still drain nearly all of the drainable water on the forming roll. We
leave just from about 1 % to about 10 %, preferably from about 1 % to about 2 %, of the
drainable water to be drained on the multiblade hydlor~il, where the vibration or pressure
pulses brought about by the blades causes a small scale ~git~tinn of the fibers until the
2s r~m~ining water is insufficient for allowing the fibers to ~ lly change their position
relative to one another. The small amount of water left at the hyllluroil is snffi~içnt for
pPrmitting agitation of the fibers to improve the form~tion, but is too small to let the vibrations
or pressure pulses deteriorate the layer purity or shake any appreciable amount of fines and
fibers out of the web.
Drainage on blades is known to be ~letrimpnt~l to layer purity and retention, but our using
the multiblade hydrofoil almost exclusively as a formation improving element and only to a
very minor extent as a draining element is a keystone of the present invention.
Process O~lh~ ion and Drainage
The balance between drainage on the forming roll and drainage on the multiblade
hydrofoil is set in the first place by the wrap angle a of the outer forming fabric on the forming
WO 94/28242 2 1 6 ~ 3 7 0 PCT/SE94/00463
roll. However, with a suction forming roll it is possible to adjust this balance to some extent by
ch~nging the vacuum level in the suction zone of the forming roll. The desired speed, basis weight,
and furnish are decisive for an u~Lhl~u~l m~gnihlcle of the wrap angle, which is set from the beginning,
but a fine-tuning of the drainage balance can be carried out by a~lju~ting the vacuum level. An
s additional adjustment of the drainage balance is possible if the headbox and the breast roll for the
outer forming fabric are mounted to be pivotable as one assembly around the rotational axis of the
forming roll in order to change the wrap angle. However, as a rule the possibilities of adjusting the
drainage balance are sufficiçnt without having to resort to complicated designs. When a suction
forming roll is used, the wrap angle is about one third of that required when a plain-s--rf~ced forming
roll is used.
DETAILED DESCRIPTION OF OTHER PREFERRED EMBODIMENTS
Figs. 11 to 14 show alL~lllalive embor~im~ntc of roll-and-blade twin wire tissue formers.
However, as these embodiments have much in common with that shown in Figs. 1 and 2 and described
above, the collc;~ ndillg items in Figs. 11 to 14 have been given ,c;r~r~.lce numerals in the 100 to 400
series, lc~ ;livt;ly. E. g. the multiblade hydrofoil, which is denoted by 7 in Fig. 1, is
dç~ign~t~d 107 in Fig. 11, 207 in Fig. 12, 307 in Fig. 13, and 407 in Fig. 14. Similarly, the
headbox that is denoted by 2 in Fig.l is rlç~ign~tçcl 102 in Fig. 11, 202 in Fig. 12, 302 in Fig.
13, and 402 in Fig. 14.
The embodiment ill--$tr~t~d in Fig. 11 differs from that shown in Fig. 1 only in that the
multiblade hy~oroil 107 is positioned on the opposite side of the fabric-web-fabric sandwich
and, cnn~equ.ontly, is located inside the loop of the inner forming fabric 104 instead of inside
the loop of the outer forming fabric. This embodiment gives the same advantages as the one
shown in Fig. 1, but may require more space in vertical direction to acco"""odate the hydrofoil
107 between the forming roll 101 and the second roll 106.
Figs. 12 and 13 show that the roll type twin wire tissue former to be modified by the
in~t~ tinn therein of a multiblade hydlofoil basically does not have to be a C-wrap former but
may as well be of a type generally known as an S-wrap former. In an S-wrap former, the
forming roll 201 or 301 is located inside a fabric loop, which in the previous embodiments was
formed by the outer forming fabric 3 but now cn~ s the inner forming fabric 203 and 303,
pe~;liv~ly, and the second roll 206 or 306 will then be located inside the loop, which in the
previous embodiments was formed by the inner forming fabric 4 but now col,~ çs the outer
forming fabric 204 and 304, ~csl!e~ y~ As illllstr~tçd in Figs. 12 and 13, the multiblade
hydrofoil 207 and 307, l~ ~;liv~1y~ is placed dowl,sll~alll of the forming roll but u~ t;alll of
WO 94/28242 PCT/SE94/00463
21 63370
14
the second roll, and inside either the outer fabric loop as shown in Fig. 12 or the inner fabric
loop as shown in Fig. 13.
Fig. 14 illustrates an embodiment, in which the roll type twin wire former shown in Fig.
11 and having a substantially vertical forming zone is modified by rotating sllhst~nti~lly the
s entire configuration on the order of 90~ so as to make the forming zone substantially horizontal
and the outer forming fabric 403 a top fabric. The multiblade hydrofoil 407 is placed inside the "
loop of the inner or bottom forming fabric 404 and between the forming roll 401 and the
second roll 406.
